Liquid-crystal display devices are now used extensively owing to their excellent characteristics (low-voltage operation; small power consumption; thin displays can be constructed; displays can be used in bright light and do not cause the viewer to suffer eyestrain). However, the most commonly used TN type display system is very slow in response compared to light-emitting display systems such as the CRT type and, further, it cannot memorize the information being displayed after the applied electric field is removed (it has no memory effect). Because of these disadvantages, many limitations have been imposed on the application of the TN type displays to a light shutter that is required to have high-speed response characteristics, a printer head, dynamic pictures that should be driven time-division-wise, such as those in TVs, etc. Therefore, the TN type displays are unsuited for these applications.
Recently, a display system employing a ferroelectric liquid crystal was reported by Mayer et al. Since both high-speed response as high as 100 to 1,000 times that of the TN type and memory effect can be obtained with the proposed display system, ferroelectric liquid crystals are expected to be a next-generation liquid-crystal display device and studies and developments are currently being made extensively.
The liquid crystal phases of ferroelectric liquid crystals belong to the chiral smectic phases of the tilt type, but from the practical standpoint, those exhibiting a chiral smectic C (hereinafter abbreviated as Sc*) phase, which is the lowest in viscosity among the chiral smectic phases, are most preferable.
A large number of liquid-crystal compounds that exhibit an Sc* phase have been synthesized and studied. For use as a ferroelectric display device, such liquid-crystal materials are preferably required: (A) that exhibit an Sc* phase over a wide temperature range including room temperature; (B) that have a proper phase sequence on the high-temperature side of the Sc* phase, with the helical pitch thereof being large, in order to obtain good orientation; (C) that have proper tilt angles; (D) that have low viscosities; and (E) that show spontaneous polarization that is strong to some degree. However, there is no known liquid-crystal compound which alone satisfies all of these.
Therefore, liquid-crystal compositions that exhibit an Sc* phase (hereinafter referred to as Sc* liquid-crystal compositions) are being used. An Sc* liquid-crystal composition is prepared by a method in which liquid-crystal compounds that mainly exhibit an Sc* phase (hereinafter referred to as Sc* liquid-crystal compounds) are mixed, or by a method in which an optically active compound or composition is added as a chiral dopant to an optically inactive liquid-crystal composition that exhibits an Sc phase (hereinafter referred to as an Sc liquid-crystal composition). However, since Sc liquid-crystal compositions have lower viscosities than Sc* liquid-crystal compositions, the latter method is suited for high-speed response and is commonly employed. Although a chiral dopant is not necessarily required to exhibit an Sc* phase as well as a liquid-crystal nature, the preferred chiral dopant is the one which, when incorporated in an Sc liquid-crystal composition, does not lower the transition temperatures of the composition to a large extent and which can induce a sufficiently strong spontaneous polarization by incorporation thereof in an amount as small as possible, from the standpoint of enabling the resulting Sc* liquid-crystal composition to have a low viscosity so as to attain high-speed response. Known optically active compounds are insufficient to constitute such a chiral dopant and, hence, there has been a need for a compound which shows stronger spontaneous polarization.
It is already known that in order that a compound can show strong spontaneous polarization, it is preferable that an asymmetric center and a dipole in the compound molecule be positioned as close together as possible and also as close to the liquid-crystal core as possible and the dipole be as strong as possible. It is further desirable that the dipole be inhibited to some degree from freely rotating on the long axis of the liquid-crystal molecule.
From the above, it is thought that a compound in which a strong dipole such as a carbonyl group, cyano group, etc. has been fixed on a ring structure and this ring structure contains an asymmetric center therein is desirable in order to obtain strong spontaneous polarization. As such a compound, however, only a compound containing a slightly optically active epoxide is known (Abstracts of the 14th Symposium on Liquid Crystals, 1988, Sendai, Japan, p. 20), and there has so far been no known compound containing in its ring structure a carbonyl group which is a stronger dipole.